Piers Corbyn photographed at his office in south London
Photograph: Antonio Olmos for the Observer

When Jeremy Corbyn won the leadership of the Labour party in September he celebrated in a pub up the road from the Houses of Parliament with a thank you to supporters and a rousing rendition of the Red Flag. On the edge of the impromptu stage that day, beside Len McCluskey of Unite and John McDonnell the future shadow chancellor, was a singular figure singing with particular pride: Piers Corbyn, the new leader’s elder brother, who has, you could say, led a parallel career of stubborn principle to his younger sibling. I was in the euphoric crowd that day, and since then I’ve wondered a bit about the relationship between the brothers, what they have learned from each other. With some of this in mind I called Piers recently and asked if he might like to talk about some of that. He agreed, but typically on the basis that he would happily talk about any subject under the sun – apart from his relationship with Jeremy and the vexed question of how his leadership is going. He shares much of his brother’s mistrust of the press; recently, he suggests, the Independent published an account of an exchange that had taken place over a family Christmas dinner. That wasn’t on. What he will talk about, though, is the thing he has talked about for just about as long as he can remember: the weather.

On this very British basis we met last week in a cafe over the road from his office on Borough High Street in south London. You don’t have to speak to Piers for very long to realise that at least a couple of Corbyn family traits are indelible: the first is that intransigent rasp of a voice with faint traces of west country burr; the second is the sense that life, like politics, is best played as a long game.

Piers Corbyn has a faith, even at 68, that if he states his beliefs often enough – though it might seem no one is listening – his day will surely come. He takes comfort in this not only from the recent elevation of his brother, but also from the example of Galileo, and of John Harrison and his measurement of longitude. “It is unfortunately not the case that the truth always prevails in the short term,” he tells me, with some conspiratorial conviction, “there are often vested interests to prevent it.”

The particular “truth” that Corbyn has in mind is perhaps the least fashionable proposal currently available in enlightened debate: it is the belief that changes in the Earth’s climate and its weather are dictated primarily by cyclical activity on the surface of the sun (and not, pointedly, by the effects of carbon dioxide in the atmosphere). It is a cause even more thankless than the return of 1980s socialism to British parliamentary politics seemed (until very recently). Corbyns have little fear of pariah status, though. What of the “settled opinion” of climate scientists and governments around the world, sealed in the recent Paris accord? “For one thing science doesn’t do settled opinions,” Corbyn says. “And for another they are all wrong.”

Thatcher probably knew climate change was nonsense. She was a scientist. But she was also a politician

This contrarian spirit began in Piers Corbyn at the age of five when his father, an electrical engineer, encouraged him to investigate the efficacy of a Victorian dew pond (he found it to be limited); it continued with his construction of all manner of meteorological equipment in the family garden in Shropshire; then through the study of tiny anomalies in the true orbit of the Earth (based, aged 17, on weekly measurements of the elevation of the sun, with findings published in the journal of the Royal Geographical Society) and into academic degrees in solid state physics and astrophysics at Imperial College London. The culmination of this obsession was a personal epiphany: “I was thinking of using the weather to discover what the sun had been doing in the past,” Corbyn says. “But then I thought, that’s just stupid: why not use the sun to predict what the weather will do in the future?” In his mind, the only thing that has got in the way of him making his fortune from that ambition, is politics.

To begin with, he says, his solar-based forecasts and calculations were way off, and he had as good as given up, but in 1984 a singular event changed the course of his working life. At the time, Corbyn says, of the year after his brother had first been elected a Labour MP, his own political loyalties were in a certain state of flux. He had been a radical student union president in London and sometime revolutionary socialist (a past that he believes would have prevented him working at the Met Office, still part of the Ministry of Defence, even had he wanted to). In the early 1980s he had been affiliated to the International Marxist Group, after a brief flirtation with the “quasi-Trotsky anarchist group, The Big Flame, for the 11 days it existed”. At the beginning of 1984, knowing his meteorological background, the IMG asked him to forecast what the winter was going to be like at the end of that year. The miners were planning a strike and “they wanted to know whether it was going to be cold enough for the strike to be effective, to bring Britain to its knees,” Corbyn recalls.

He said no, but under pressure from his comrades he went back to the data, “looked at a lot of new relationships and correlations” and came to the conclusion that, yes, it was going to be a very cold winter. He presented his evidence to the Miners’ Educational Support Committee (“I think it was called that. We had a lot of committees…”) And it was passed on up to the NUM. As the strike unfolded Corbyn’s prediction proved correct, the winter of 1984-85 was unusually cold.

Was the NUM grateful?

“Very,” he says. He had phone calls from head office, “not [Arthur] Scargill personally but one of his senior assistants”, asking how long the cold weather would last. He told him he thought it could possibly last long enough to bring down the Thatcher government, but by March the strike had collapsed and he never got the chance to be proved right.

All the while Corbyn was assessing the proximity of revolution by monitoring the electricity output at Goldsmith’s college where he was a lecturer in the physics department. “The voltages were below the legal minimum all that winter,” he says. “It is supposed to be 240v, it was below 210v and for safety they are legally bound to turn it off. The government were desperately doing everything they could to conserve power. I called to say I had measured the voltages at the University of London and they were below that minimum. They said, ‘Well, Mr Corbyn you must have faulty equipment,’ and put down the phone.”

Piers Corbyn’s brother, Jeremy Corbyn celebrates his victory in the Labour leadership campaign, September 2015. Piers joined him in a rendition of The Red Flag. Photograph: Katherine Anne Rose for the Observer

It was in those weeks and months, Corbyn suggests, that Margaret Thatcher came up with her most devious plan to deindustrialise Britain and defeat the miners once and for all: she would popularise and endorse the science of man-made climate change, as a way of converting Britain from coal to nuclear power. The Hadley Centre, the world’s first dedicated research institution became her pet project. Thatcher, who later recanted her doom-mongering, “probably knew climate change was nonsense,” Corbyn suggests, “she was a scientist. But she was also a politician. The rest is history.”

Corbyn sees that moment as the one in which politicians first saw the political capital to be made from prophesying the coming apocalypse. Worse though, in his mind, the most vehement expression of that prophesy was not on the right but on the left. Climate change became a progressive cause.

For Corbyn, it answered the crisis on the left created by the fall of the Berlin Wall. “The left needed a new totem pole. Something for the young to dance around and feel good about,” he says, somewhat bitterly. “Revolutions had failed. Russia had failed. Socialism wasn’t dead but it needed a cause. The Red-Green alliance became that cause. It needed massive state spending, which they equated with socialism. In my view the left stopped thinking in 1989. It is only my brother who has started to change the agenda of privatisation and so on.”

It is hard to imagine that Piers has not bent his brother’s ear with elements of this analysis at family gatherings for about 25 years. Does he show signs of wavering?

“I don’t speak for my brother on any of this,” he says. “I don’t give advice to him where it is not appropriate.”

Did Jeremy share his obsession with the weather as a kid?

“Not really,” he says. “We were all different. Our eldest brother was into cars. He became a flight test engineer for Concorde. The next brother is a mining engineer, went to Imperial college. Then there is me, scientist. Then Jeremy, who was always politics. I think we all took after different aspects of our dad; while my mother encouraged us to make things all the time. The house was rundown and we boys had to do repairs. Jeremy and I learned how to repoint brickwork and so on. We became pretty good builders.”

The left stopped thinking in 1989. It is only my brother who has started to change the agenda

Piers has had perhaps less convincing success with the construction project that has occupied most of his adult life. After the relative triumph of his forecast for the miners he worked pretty much full-time on his long-range models. By 1989, the first year he heard of the idea of man-made climate change, he was ready to sell his formula to the world.

Ignoring the traditional model of publication and peer review, ostensibly to prevent others stealing his ideas, he engaged the bookmaker William Hill to construct some weather bets for him with the help of the Met Office. “That went on for 12 years,” he says. “I had 30 bets at £20 each on long-range aspects of the weather each month; betting the average temperature would be below normal, that kind of thing. We made a lot of money that way.”

How much is a lot of money?

“I made about 40% profits on stake, overall,” he claims. “I would bet £600; I would get an extra £200 back. It would pay the rent basically.” He and his girlfriend at the time no doubt spent a lot of time looking anxiously at the sky.

After this marginal success, a little better than putting a wet finger in the wind, some commercial clients came calling. For a while, Corbyn exclusively told the clothes store Etam the optimum time to sell its T-shirts or its hats and scarves. Some farmers were seduced by his predictions of storm and drought. And then the scientific consensus around carbon dioxide as principal climate-changer hardened, and Corbyn’s company, WeatherAction, which floated on the stock exchange in 1997, went back into private ownership after two years.

Adversity has only stiffened Corbyn’s resolve, however. Despite his marginal status he is fighting global opinion on all fronts. These days he does the odd presentation for Texan oil companies, or Russian climate sceptics. And he has not given up on a shift of opinion closer to home.

“Whatever you think of it all, the net result of these beliefs has been a deindustrialising of the west and the shift of carbon dioxide production from Britain to India and China,” he says. “Labour under Jeremy are now saying we have got to make a special case for steel and various industries that have to pay all these significant taxations on energy costs, which is the reason a lot of them are closing down.”

Piers is whispering in the ear of any MP who will listen: “Graham Stringer, of Labour, is sympathetic,” he says, “Sammy Wilson DUP, a number of Tories, Boris is very sympathetic [Johnson tends to use Corbyn, in an unlikely alliance, as his climate guru]. Now I think we have George Galloway which is significant, because he can be very persuasive…”

Inevitably, Corbyn has arrived for coffee armed with graphs and charts; so before I go I try to get him to explain how his calculations work. He talks 10 to the dozen about solar magnetic fields and lunar modulation and stratospheric winds. He outlines his database of 200-year-old German weather maps and solar observation from American satellites. “The principle is that all weather that happens has happened before, all storm systems, the question is finding the closest match…”

If the results are as good as he claims, I suggest, why haven’t other people picked up on his system?

“We haven’t told them how it’s done!” he says. “We had the Solar Weather Technique, ‘SWT zero’. We now have now got the Solar Lunar Action Technique, ‘SLAT’, and we are on version 12CS – s is for special. We have gone through 20 improvements and understandings on this…”

As he talks I have the impression of an army of researchers tracking and calculating, poring over maps in order to compete with the Met Office’s supercomputer.

When you say “we…” I ask.

“I mean me,” he says.

“OK” I say. “And what does the system suggest we can look forward to?

“It will get much, much colder,” Corbyn asserts (he has lately suggested that in the longer term we are on the verge of a great cooling cycle, which will vindicate him once and for all). “Next month we have a displaced polar vortex situation. Plenty of snow coming.”

We both glance out at the spring-like day on Borough High Street.

“It’s got a chance, I guess,” I say. Corbyn grins. As he knows only too well, stranger things have happened.